Controllable electronic attenuator having zero differential phase shift



J. G. MORRISbN 3,539,909

CONTROLLABLE ELECTRONIC ATTENUATOR HAVING ZERO DIFFERENTIAL PHASE SHIFTNov. 10, 1970 Filed Jan. 28, 1969 2 Sheets-Sheet 1 FIG.5. I FIG-.6.

.r/v l/E/V 70/? Jowv 6. Mom/50m Nov. 10, 19 0 J. G. MORRISON 3,539,909CONTROLLABLE ELECTRONIC ATTENUATQR HAVING ZERO DIFFERENTIAL PHASE SHIFTFiled Jan. 28. 1969 2 Sheets-Sheet 2 PRIOR ART PRIOR ART PRIOR ART M R21 i 10 l I/V l/E/VTOR J0H/v 6. MORE/SON r 'ATTOR/VE) United StatesPatent O CONTROLLABLE ELECTRONIC ATIENUATOR HAVING ZERO DIFFERENTIALPHASE SHIFT John G. Morrison, Syosset, N.Y., assignor to Sperry RandCorporation, a corporation of Delaware Filed Jan. 28, 1969, Ser. No.794,539 I Int. Cl. H02j 3/18 US. Cl. 323-111 5 Claims ABSTRACT OF THEDISCLOSURE An electronic attenuator circuit comprising a voltage dividerhaving a switch connected thereto for controlling the voltage ratiobetween the input and output terminals of the divider and operating inconjunction with a diode and resistor network energized by first andsecond Voltage sources for forward and reverse biasing the diodeaccordingly as the control switch is opened and closed thereby causingthe output signal to have the same phase shift relative to the inputsignal for both states of the switch.

"BACKGROUND OF THE INVENTION I {The present invention relates tocontrollable electronic attenuator circuits comprising a voltage dividernetwork having a switch connected thereto for adjustingthe voltagedivision. between the input and output terminals of the attenuator andmore particularly to means for eliminatingv dilferential phase shiftbetween the input and output voltages for both the open and closedstates of the switch. I v v In prior art electronic attenuator circuitswherein a switch connected to a voltage divider network is opened andclosed to control the amplitude of the signal at the Output terminal ofthe attenuator relative to the signal amplitude applied to its inputterminal, the phase of the output signal is dependent on whether theswitch is opened or closed. This occurs because an inherent capacitanceexists across the open contacts of any switch. Consequently, when theswitch contacts are open, itsswitch capacitance forms part of theattenuating circuit whereas when the switch is closed its capacitance isremoved from the circuit. The differential phase shift'which isintroduced in the course of operating the attenuator 'is undesirable andin some instances intolerable especially, for example, where preciseamplitude control is required or the load connected to the attenuator isan'amplifier stage which isse'nsitive to the phase of the signal appliedto its input terminal.

SUMMARY on THE, INVENTION I @The. present invention eliminates phaseshift caused by a switching mechanism ,in controllable attenuatorcircuits by the provision of a circuit comprising a diode and two'resistors which are connected to the attenuator and arranged to operatein a manner such that the switch is essentially removed from thecircuitwhen its contacts are open circuited. A preferred embodiment of theinvention includes a basic attenuator circuit comprising the seriescombination of two impedanceelements and a switch having one of itscontacts connected to ground. An input signal referenced to ground isapplied across the full network and the output signal, which is also icereferenced to ground, is obtained at the junction of the two resistors.A diode interconnects the input terminal with the ungrounded switchcontact and individual resistors couple each side of the diode to adiscrete voltage source. The resistors :and voltage sources operate toreverse bias the diode 'when the switch is closed whereupon the outputvoltage is attenuated simply in accordance with the ratio of theimpedances. On the other hand, when the switch is opened and itsinherent capacitance is present in the circuit, the diode is forwardbiased thereby coupling the input terminal directly to the ungroundedswitch contact. As a result, the switch is etfectively removed from thecircuit when its contacts are opened. Hence, the phase of the outputsignal relative to the input signal is the same for both the open andclosed states of the switch. Moreover, as will be discussed more fullyin the subsequent detailed description, the invention provides the samerelative phase shift for both states of the switch irrespective of thenature of the load which is connected to the attenuator. Thus, a vacuumtube or transistor amplifier stage having a high input capacitance willnot introduce relative phase shift between the attenuating andnon-attenuating modes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described withreference to the following drawings in which similar components areidentified by the same numeral and letter designations.

FIG. 1 is a circuit diagram of the preferred embodi ment of theinvention;

FIG. 2 is a circuit diagram of a prior art attenuator circuit;

' FIGS. 3 and 4 are equivalent circuits of the prior art circuit of FIG.2 for the condition where the attenuator control switch is closed andopened respectively; and

FIGS. 5 and 6 are equivalent circuits of the preferred embodiment ofFIG. 1 for the condition where the attenuator control switch is closedand opened respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT S Capacitor C is not an actualcapacitor connected across switch S but represents the inherentcapacitance existing across the collector-to-emitter terminals of thetransistor when it is in a non-conducting state. Obviously,

other kinds of switches could be used in place of the transistor switch.In any case, capacitor C would represent the capacitance across the openterminals of the switch. As previously mentioned, the function of theinvention is to eliminate phase shift caused by both this switchcapacitance and capacitance inherent in load 14 connected through D.C.blocking capacitor C to the attenuator output terminal 13 at thejunction of resistors R and R Blocking capacitors C and C haverelatively large capacitance so as to present a very low impedance tothe AC. input signal. The load is a transistor amplifier comprising annpn transistor 15 having its collector terminal 16 connected throughload resistor R to power supply B+ which also connects through biasresistor R to base terminal 17 to provide the required base currentthrough the base-to-emitter terminal 18 coupled to ground 22. CapacitorC as in the case of capacitor C is not an actual component butrepresents the input capacitance appearing across the base-to-emitterand base-to-collector terminals of the amplifier. It should beunderstood that any kind of load, represented typically as an impedanceZ can be connected to the attenuator although it is expected that theload will generally be an amplifier stage of some sort employing eithertransistors or vacuum tubes and therefore will have a comparativelylarge reactive component in its input impedance.

Before proceeding to a detailed discussion of the operation of theinvention, consider the prior art attenuator shown in FIG. 2. Whentransistor switch S is driven into saturation by a negative voltageapplied to its base terminal 19, the impedance between the collector 20and emitter 21 is very small so point 23 is elfectively connected toground 22. Moreover, since blocking capacitors C and C havecomparatively large capacitance they may be regarded as short circuitsfor the purpose of A.C. analysis. Hence, the equivalent circuit for theattenuator with switch S closed is as shown in FIG. 3 whereupon thevoltage E at the output terminal 13 of the attenuator is related to theinput voltage E by the ratio of the resistances of resistors R and Rthat is,

Since no reactive terms appear in this equation, it is seen that theoutput voltage is in phase with the input voltage. In thenon-attenuating mode, on the other hand, switch S is open(non-conducting) so that the equivalent circuit of the attenuator is asshown in FIG. 4. In this instance, the switch capacitance C appearingacross the output terminal to ground causes output voltage E to be phaseshifted relative to B Now refer again to FIG. 1 for an understanding ofhow the invention provides for the output voltage to have the same phaserelative to the input voltage for both states of the switch. For theattenuation mode, where switch S is closed by means of a negative signalapplied to base terminal 19, point 23 is essentially connected to groundand voltage source V causes electrons to flow through resistors R R andR to produce a negative potential at point 24. This reverse biases diodeD so that it presents an open circuit to the A.C. input signal whereuponthe A.C. equivalent circuit of attenuator 12 is as shown in FIG. 5. Itis thus seen that the input voltage E appears directly across R and isthen applied to R and R with the result that the output voltage isdetermined by the ratio of the resistances of resistors R and R as inthe case of the prior art attenuator.

In the non-attenuation mode with the negative signal removed from baseterminal 19, S is open and electrons flow from source -V which is morenegative than source -V through resistors R R R and R and therebyestablish a potential at point 23 more negative than the potential atpoint 24 so that diode D is forward biased and presents a short circuitto the A.C. input signal. Under this condition, input signal E isconnected directly to point 23 as shown in FIG. 6. As a result, 15;appears directly at the attenuator output terminal 13 so that the outputsignal is in phase with the input signal exactly as it was when theswitch was closed.

For the case where the switch is closed (FIG. 5) the Thevenin impedancelooking from output terminal 13 back toward A.C. source is simply R inparallel with R or and the A.C. Thevenin voltage at output terminal 13is The ratio of this Thevenin voltage to the Thevenin resistance is E /Rwhich is the same as the ratio of the voltage to the resistance lookingfrom output terminal 13 back toward A.C. source 10 in the circuit ofFIG. 6 where the switch is open. Thus, a reactive load element connectedto the attenuator will produce the same relative phase shift whether theswitch is open or closed. This result does not obtain for the prior artattenuator since capacitor C is present in this circuit when the controlswitch is open.

An attenuator constructed in the aforedescribed manner has utility overa range of frequencies extending from only a fraction of a cycle up totens of megacycles and it will be apparent to those skilled in the artthat the invention also has utility with voltage divider attenuatingcircuits including more than two impedance elements or configurations inwhich the switching mechanism is connected in parallel with one or moreof the impedances. Moreover, it will be recognized that the voltagedivider can be constructed of reactive components as well as resistors,the essential requirement being that like impedance elements, that is,all resistors, all capacitors, or all inductors must be used.

While the invention has been described in its preferred embodiment, itis to be understood that the Words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

What is claimed is: v

1. An electronic attenuator circuit comprising (a) a voltage dividerincluding a plurality of like impedance elements,

(b) an input terminal connected to the divider for applying analternating current input signal thereto and an output terminalconnected to the divider for providing an output signal proportional tothe input signal,

(c) a switch connected to the voltage divider for controlling theproportion of the input signal transmitted to the output terminal inaccordance wtih Whether the switch contacts are open or closed,

(d) a unidirectional current conductive device interconnecting the inputterminal and one switch contact,

(e) a first resistor connected at one end to the input terminal andadapted to be connected at the other end to a first voltage source forbiasing the unidirectional current conductive device such that the inputsignal is precluded from passing therethrough when the switch contactsare closed, and

(f) a second resistor connected at one end to said one switch contactand adapted to be connected at the other end to a second voltage sourcewhich provides a voltage greater than that provided ,by the first sourceand thereby operates to bias the unidirectional current conductivedevice such that the input signal is transmitted therethrough when theswitch contacts are open.

2. The apparatus of claim 1 wherein the switch is operative when closedto adjust the amplitude of the output signal to a level which is afraction of the amplitude of the input signal and operative when open toadjust the amplitude of the output signal to be equal to the inputsignal.

3. The apparatus of claim 1 wherein a signal source connected to theinput terminal is connected in parallel with the open contacts of theswitch when the unidirectional current conductive device is forwardbiased whereby the attenuator circuit presents equivalent sourceimpedances for both states of the switch with respect to a loadconnected to the output terminal.

4. The apparatus of claim 1 wherein the impedance elements are connectedin series to one switch contact and the other switch contact isconnected to a reference point,

minal connected to the junction therebetween, the unidirectional currentconductive device is a diode and the switch is a transistor having base,emitter and collector terminals, the base terminal being adapted forconnection to a potential source for driving the transistor beweencut-ofi? and saturation accordingly as the potential source 10establishes a reverse and forward bias across the base-toemitterterminals.

References Cited UNITED STATES PATEN lS 3/1968 Metcalf. 7/1969 Collings.

I D MILLER, Primary Examiner G. GOLDBERG, Assistant Examiner US. Cl.X.R.

